Pretreatment Before Powder

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From: Products Finishing, David S. Peterson

Posted on:12/1/2008

A customer has just purchased a powder coating plant. The steel is being stored outside and is very rusty prior to powder coating. Please provide recommendations for pre-treatment prior to powder coat.

Q. A customer has just purchased a powder coating plant. The steel is being stored outside and is very rusty prior to powder coating. Please provide recommendations for pre-treatment prior to powder coat. Regards C.K.

A. Without knowing your current capabilities or pretreatment process, it is difficult to provide a specific recommendation. In general, I would suggest that money spent up-front on steel with a rust inhibitor and protected storage may be well spent and significantly cheaper in the long run than installing a rust removal procedure to treat all the steel.

From your description, it sounds like all steel will need to spend a significant amount of time in a sulfuric acid pickling tank to remove the rust prior to pretreatment. This will then require rinsing and possibly the application of a rust inhibitor unless the metal will be going directly to pretreatment. The added cost of the tanks, ventilation, waste treatment along with the ongoing expense of chemicals, depreciation, energy, floorspace and labor should be enough to drive the decision to purchase the steel clean and with a rust inhibitor.

A light and occasional rusting of steel parts could be removed by addition of a phosphoric acid pretreatment stage. Assuming a phosphate pretreatment system, this should be located between the initial alkaline cleaning and the phosphate steps. This would be important to remove grease and oils such that the phosphoric acid could more effectively work on the entire surface. Otherwise, the phosphoric acid will only go after the areas that do not contain inhibitor, resulting in a spotty pretreatment.